（二）PCB board size
The thickness of the PCB Board shall be determined according to the functions of the PCB board, the weight of the components installed, the specifications of the PCB board socket, the size of the PCB board and the mechanical load to be borne. We should ensure enough stiffness and strength. The thickness of common PCB boards is 0.5mm, 1.0mm, 1.5mm and 2.0mm.Considering the cost, the length of copper film line and the ability to resist noise, the smaller the size of the PCB board, the better. However, the size of the PCB plate is too small, and the heat dissipation is poor. The production cost of PCB is related to the area of PCB. The larger the area is, the higher the cost is. When designing a PCB board with a chassis, the size of the PCB plate is also limited by the size of the enclosure. It is necessary to determine the size of the chassis before determining the size of the PCB board, otherwise the size of the PCB plate can not be determined. In general, the specified wiring range in the forbidden wiring layer is the size of PCB board.The best shape of the PCB board is rectangle, and the aspect ratio is 3:2 or 4:3. When the size of the PCB plate is larger than 200*150mm, the mechanical strength of the PCB plate should be considered. In a word, the size of PCB should be determined by considering the advantages and disadvantages.
（三）PCB component layout
Although Protel DXP can layout automatically, in fact, the component layout of PCB is almost done by hand. The layout of PCB components generally follows the following rules:
1. Layout of special components
The layout of special components shall be considered from the following aspects:
1) high frequency components
The shorter the wires between the high frequency components, the better. Try to reduce the distribution parameters of the wires and the electromagnetic interference between them. The easily disturbed components cannot be too close. The distance between the components that belong to the input and the components that belong to the output should be as large as possible.
2) components with high potential difference
The distance between the components with high potential difference and the connecting wire should be increased to avoid damaging the components in case of accidental short circuit. In order to avoid the occurrence of creepage, it is generally required that the distance between the copper film wires of 2000V potential difference should be greater than 2mm. If the potential difference is higher, the distance should be increased. Devices with high voltage should be placed in places that are not easy to touch when debugging.
3) too heavy components
Such elements should be fixed with brackets, and for heavy and heavy calorific elements, it should not be installed on the PCB board.
4) heating and thermosensitive elements
Pay attention to keep the heating element away from the thermal element.
5) components that can be adjusted.
For the layout of adjustable elements such as potentiometer, adjustable inductance coil, variable capacitance, microswitch, etc., the structural requirements of the whole machine shall be taken into consideration. If it is adjusted inside the machine, it shall be placed on the PCB board where it is easy to adjust. If it is adjusted outside the machine, its position shall correspond to the position of the adjusting knob on the panel of the chassis.
6) circuit board mounting hole and bracket hole
The installation holes of the PCB board and the mounting holes of the bracket should be reserved, because these holes and holes can not be cabled nearby.
2. Layout according to circuit function
If there is no special requirement, try to arrange the components according to the component arrangement of the schematic diagram. The signal enters from the left, outputs from the right, inputs from the top and outputs from the bottom. According to the circuit process, the location of each functional circuit unit is arranged to make the signal flow more smoothly and keep the same direction. With each functional circuit as the core, the layout of components should be uniform, neat and compact. The principle is to reduce and shorten the leads and connections between components. The digital circuit part should be separately arranged from the analog circuit part.
3. Distance between components and PCB edge
All components shall be placed within 3mm from the edge of PCB, or at least the distance from the edge of PCB is equal to the thickness of the board. This is because the assembly line plug-ins and wave soldering in mass production shall be provided to the guide rail groove for use, and it is also to prevent the edge of PCB from damage due to the shape processing, which will cause the copper film line to break and lead to waste products. If there are too many components on the PCB, which must not exceed 3mm, add 3mm auxiliary edge on the edge of PCB, open V-shaped groove on the auxiliary edge, and break it off by hand during production.
4. The order of component placement.
First, place the components that are closely aligned with the structure, such as power outlets, indicator lights, switches and connection plug-ins. Then place special components, such as heating elements, transformers, integrated circuits, etc. Finally, place small components, such as resistance, capacitance, diode, etc.
